Orthopedic implant having a porous surface and method of making same

ABSTRACT

An orthopedic implant has a porous surface with a plurality of pores, some of which are provided with a pores-within-a-pore structure. The pores-within-a-pore structure has a pore opening ranging in size between 10 and 800 microns. The present invention also discloses a process for making an orthopedic implant which is provided in the surface thereof with a pores-within-a-pore porous structure.

FIELD OF THE INVENTION

[0001] The present invention relates to an orthopedic implant having aporous surface and a method for making such an orthopedic implant.

BACKGROUND OF THE INVENTION

[0002] The porous surface of the orthopedic implant is generallyprovided with a plurality of pores ranging in diameter between 10 and500 microns. The bone tissues are thus capable of growing in the poresso as to unite the implant with the bone tissues. The conventionalmethods for making the implant with a porous surface include the plasmaspray process, the sintering process, and the diffusion bonding process.The pores formed by the plasma spray process are not in communicationwith the outside, as shown in “A”, “B”, pores of FIG. 1. As a result,the bone tissues can not grow into them. On the other hand, they tend toform boundaries which are susceptible to breakage. The boundaries referto the interfaces between the pores and the bone tissues. Similarly, thepores formed by the sintering process are not in communication with theoutside and are susceptible to poor fatigue strength. As a result, theplasma spray process and the sintering process (either sintered beads orsintered fiber metal) can not be used to make the main structure of theorthopedic implant (Richard J. Friedman, et al., entitled “CurrentConcepts in Orthopaedic Biomaterials and Implant Fixation”, The Journalof Bone and Joint Surgery, Vol. 75-A, No. 7, July 1993). The diffusionbonding process, such as vapor deposition techniques for making theporous tantalum structure, can be used to make pores free from thedrawbacks as described above [J. Dennis Bobyn, Michael Tanzer, and Jo E.Miller: Fundamental Principles of Biologic Fixation. In Morrey BF (ed):Reconstructive Surgery of The Joints. Churchill Livingstone, 1996, pp75-94]. However, such porous structure has a relatively poor bendingstrength and is therefore vulnerable to deformation or damage by abending force exerting thereon. Moreover, the porous titanium structureis not cost-effective.

SUMMARY OF THE INVENTION

[0003] It is the primary objective of the present invention to providean orthopedic implant having a porous surface.

[0004] It is another objective of the present invention to provide anorthopedic implant having a pores-within-a-pore porous surfacestructure.

[0005] It is still another objective of the present invention to providean orthopedic implant having a pores-within-a-pore porous surfacestructure, with a pore opening ranging in size between 10 and 800microns.

[0006] It is still another objective of the present invention to providea method for making an orthopedic implant having a porous surface.

[0007] The present invention provides an electrochemical techniqueemploying a large electric current density and/or an intermediateelectric current density for forming a pores-within-a-pore porousstructure on a metal surface such that the porous structure is incommunication with the outside, and that the porous structure iscircumvented by a metal substrate. As a result, the porous structure isnot vulnerable to boundary severance, poor fatigue strength and/orbending.

BRIEF DESCRIPTION OF THE DRAWINGS

[0008]FIG. 1 shows an optical microscopic picture of a hip stem made bythe conventional plasma spray process.

[0009]FIG. 2 is a scanning electron microscopic (SEM) picture of anorthopedic implant made in Example 1 of the present invention.

[0010]FIG. 3A is another SEM picture of the orthopedic implant made inExample 1 of the present invention.

[0011]FIG. 3B is a plot of FIG. 3A labeling pores of thepores-within-a-pore structure.

[0012]FIG. 4 is a SEM picture of an orthopedic implant made in Example 2of the present invention.

[0013]FIG. 5 is another SEM picture of the orthopedic implant made inExample 2 of the present invention.

[0014]FIGS. 6A to 15A are SEM pictures of the orthopedic implants madeby Examples 3 to 12 of the present invention, respectively.

[0015]FIGS. 6B to 15B are optical microscopic pictures of the orthopedicimplants made by Examples 3 to 12 of the present invention,respectively.

DETAILED DESCRIPTION OF THE INVENTION

[0016] The orthopedic implant of the present invention has a poroussurface with a plurality of pores, some of which are pores-within-a-poreporous structures. At least some of the pores-within-a-pore porousstructures have an opening ranging in diameter between 10 and 800microns, preferably between 50 and 600 microns, and more preferablybetween 100 and 500 microns.

[0017] The examples of the orthopedic implants are hip stem, screws,fixation rods, hooks, cages, etc. The orthopedic implants are made ofany conventional biologically compatible material, such as the unitarymetal, alloy, or metal or nonmetal coated with a metal layer (unitarymetal layer or alloy layer) or with multiple metal layers.

[0018] The above orthopedic implants according to the present inventionhave at least the pores-within-a-pore porous structure for boneingrowth, as well as the conventional non-pores-within-a-pore porousstructure on their surface.

[0019] The pores-within-a-pore porous structures of the presentinvention have pores (the first-stepped pores), within some of whichthere are pores (the second-stepped pores), within some of thesecond-stepped pores there are pores (the third-stepped pores), and soon even up to sixth-stepped pores. The pores-within-a-pore porousstructures of the present invention may comprise any combinations ofdifferently stepped pores, such as the first-stepped pores; thefirst-stepped pores within which there are only the second-steppedpores; the first-stepped pores within which there are the second-steppedpores and the third-stepped pores; the first-stepped pores within whichthere are the second-stepped pores, the third-stepped pores and thefourth-stepped pores, and so on. Within the same first-stepped pore mayhave one or more second-stepped pores, and within the samesecond-stepped pore may have one or more third-stepped pores, and so on.

[0020] The above pores may have any shape, which may be a regulargeometric form, such as circular, cylindrical, and oval; or irregular.

[0021] The pore opening used herein, unless indicated otherwise, refersto the opening of the first-stepped pore facing outward (away from thesubstrate). If the stepped pore has an opening which is round orcircle-like, the size of the opening refers to the circle diameter. Ifit is irregular in shape, the size of the opening refers to the averageof the maximum distance and the minimum distance of the opening.Similarly, the opening of the two-stepped pore is referred to as thetwo-stepped opening. The opening of the three-stepped pore is referredto as the three-stepped opening, etc.

[0022] The high-stepped (two-stepped, three-stepped, four-stepped . . .) openings of the pores-within-a-pore porous structure of the presentinvention have the size ranging between 10 and 500 microns, preferablybetween 50 and 400 microns, and most preferably between 50 and 300microns.

[0023] The number of the pores-within-a-pore porous structures have apore opening ranging in diameter between 10 and 800 microns ispreferably over 30% of the total number of said pores-within-a-poreporous structures, more preferably over 50%, and most preferably over80%.

[0024] The high-stepped pores having a pore opening ranging between 10and 500 microns are preferably over 20% of the total number of thehigh-stepped pores of the pores-within-a-pore porous structures, morepreferably over 40%, and most preferably over 70%.

[0025] The number of the first-stepped-pores within which there is nohigh-stepped pores is less than 70% of the total number of saidplurality of pores, preferably less than 50%, and more preferably lessthan 40%.

[0026] The above pores, either the first-stepped pores or thehigh-stepped pores, may be superimposed. For example, a plurality offirst-stepped pores are partially superimposed such that thesuperimposing portions form the second-stepped pores, and within thefirst-stepped pores two or more second-stepped pores are superimposed,and thus forming three-stepped pores in the superimposing portion.

[0027] Any conventional methods may be used to control on which parts ofthe surface the pores-within-a-pore porous structure. For example, aetching-resistant layer may be coated or a Teflon® sealing tape may beput on the surface on which no pores-within-a-pore porous structure isto be formed during the formation of the pores-within-a-pore porousstructure.

[0028] The process for making the orthopedic implant having a poroussurface according to the present invention comprise:

[0029] forming a plurality of initial pores on at least one portion of asurface of an orthopedic implant having; and

[0030] forming a pores-within-a-pore structure within the plurality ofinitial pores,

[0031] wherein an intermediate electric current density pore etchingmethod is used to form said pores-within-a-pore structure.

[0032] The definitions of the above orthopedic implant, thepores-within-a-pore porous structure, the pore opening and the size ofthe pore opening are the same as described above.

[0033] A suitable method for forming said plurality of initial pores onat least one portion of the surface of the orthopedic implant may be theconventional methods, such as the laser drilling, the high pressurewater drilling, chemical pore etching, and the electrochemical poreetching method described below. The initial pores are substantiallyfirst-stepped pores having an opening ranging between 0.2 and 500microns, preferably between 0.5 and 200 microns, most preferably between1 and 100 microns. The initial pores may contain one or morehigh-stepped pores formed by the method of the present invention.

[0034] Preferably, the method for forming said plurality of initialpores on at least one portion of the surface of the orthopedic implantmakes use of the electrochemical pore etching method, and morepreferably the high electric current density pore etching method, inwhich the orthopedic implant is submerged in an electrolyte solution tocarry out the electrolysis by an electric current density higher than0.2 A/cm². The electrolysis is carried out by the conventional methodsin existence, such as the three-electrode potentiostat electrolysis,two-electrode potential control electrolysis, the pulse electrolysis,and the galvanostat electrolysis, etc.

[0035] The above electrolyte solution may be a solution containingsupporting electrolyte, or preferably a solution containing anelectrolyte capable of etching pores, such as HCl, NaCl, NaF, HF, etc.,preferably NaCl. The electrolyte solution containing 1-8% by weight ofNaCl, or preferably 2-5% by weight of NaCl, is recommended.

[0036] As far as the high electric current density pore etching methodis concerned, the electrolysis conditions are adjustable depending onthe nature of the orthopedic implant, provided that the electric currentdensity is greater than 0.2 A/cm², more preferably 0.3 A/cm², and mostpreferably 0.5 A/cm². The electrolysis conditions include (but notlimited to) electrolyte concentration, electrolysis temperature,electrolysis time, electrolysis potential (the potential between thereference electrode and the working electrode in the three-electrodepotentiostat electrolysis; and the applied potential between the anodeand cathode).

[0037] Generally speaking, the conductivity of the electrolyte solutionis preferably greater than 10 S, and more preferably not less than 20 S,so as to attain the electric current density greater than 0.2 A/cm². Thecorrelation between the conductivity and the electrolyte concentrationof the electrolyte solution can be found in the Chemistry or ChemicalEngineering Handbooks.

[0038] In order to facilitate the high electric current densityelectrolysis, an elevated electrolysis temperature is preferable, but isnot to exceed 80° C. to avoid undesired vigorous reaction. A suitableelectrolysis temperature will be ambient temperature to 60° C., andpreferably to 50° C.

[0039] The time of the high electric current density electrolysis rangesbetween 30 seconds and 5 minutes, preferably between 1 and 4 minutes,most preferably between 1 and 3 minutes, for formation of the initialpores with the size of opening thereof being in the range of 0.2-500microns.

[0040] Said intermediate electric current density pore etching methodused in the process of the present invention is similar to the highelectric current density pore etching method, except that the electriccurrent density ranges between 0.05 A/cm² and 1 A/cm², preferablybetween 0.1 A/cm² and 0.8 A/cm², most preferably between 0.2 A/cm² and0.7 A/cm²; and the electrolysis time preferably ranges between 3 and 30minutes, and more preferably between 5 and 20 minutes.

[0041] The intermediate electric current density pore etching method maybe same or different from the high electric current density pore etchingmethod in electrolyte solution, and electrolysis temperature, butpreferably same in electrolyte solution, and electrolysis temperature.

EXAMPLE 1

[0042] A rectangular cage having a surface area of about 3.5 cm² waselectrolyzed for 2 minutes in 3.5 wt % NaCl electrolyte solution at roomtemperature by a constant current of 2.0 A/cm². Thereafter, a constantelectric current of 1.0 A/cm² was used to carry out the electrolysis for5 minutes. The results are shown in the scanning electron microscopic(SEM) pictures of FIGS. 2 and 3A. As shown in FIG. 2, the cage isprovided with the pores-within-a-pore porous structure. Referring toFIGS. 3A and 3B, P1, P2, P3, P4, P5, and P6 are all first-stepped poreswith the pore openings being respectively about 450 μm, about 500 μm,about 320 μm, about 350 μm, 800 μm. P11, P21, and P51 are second-steppedpores within the first-stepped pores P1, P2, and P5, respectively, andthe pore opening of the second-stepped pores P11, P21, and P51 are about225 μm, about 270 μm, and about 150 μm, respectively. P31 is a commonsecond-stepped pore of the first-stepped pores P3, P4 and P5. The poreopening of the common second-stepped pore P31 is about 150 μm. P61, P62,P63, P64 and P65 are the second-stepped pores of a first-stepped poreP61, and their opening respectively are about 225 μm, about 225 μm,about 180 μm, about 225 μm, and about 270 μm. P651 is a third-steppedpore of the second-stepped pore P65, whose opening size is about 200 μm.P6511, P6512, and P6513 are fourth-stepped pores of the third-steppedpore P651, with the sizes of openings thereof being about 100 μm, about100 μm, and about 90 μm.

EXAMPLE 2

[0043] This example uses the two-stage electrolysis procedures similarto that of the Example 1. The current density (the electrolysis time)for forming a pores-within-a-pore structure on a surface of a fixationrod in the two electrolysis stages were respectively 0.65 A/cm² (30seconds) and 0.2 A/cm² (6 minutes).

[0044] The results are shown in FIGS. 4 and 5. FIG. 4 is a scanningelectron microscope (SEM) picture taken in the direction perpendicularto the central axis of the fixation rod, in which the surface of thefixation rod is provided with the pores-within-a-pore porous structure.FIG. 5 is a SEM picture of the surface of the fixation rod showing thatthe distribution and the size of the pores are uniform, and that theoverlapping of the first-stepped pores is not serious.

EXAMPLES 3-12

[0045] A fixation rod was electrolyzed for one minute with a constantcurrent of 2.0 A in 3.5 wt % NaCl electrolyte solution at roomtemperature. Thereafter, the fixation rod was electrolyzed with aconstant current density electrolysis in accordance with the conditionslisted in Table 1. The fixation rod has a surface area of 3.14 cm². Theresults of Examples 3-12 are shown in Table 1.

[0046] The results are also shown in FIGS. 6A-15A, and FIGS. 6B-15B.FIGS. 6A-15A show SEM pictures of the surfaces of the fixation rods.FIGS. 6B-15B show the optical microscopic pictures of the surfaces ofthe fixation rods. TABLE 1 Electric Electric current Electrolysis PorePore n- current density time distribution distribution Pore stepped SEMEx. (A) (A/cm²) (min) uniformity density depth Pore picture 3 0.2 0.0645.0 X X X 1-2  6A 4 0.3 0.095 8.3 Δ Δ Δ 1-3  7A 5 0.4 0.127 6.25 0 Δ Δ2-3  8A 6 0.5 0.159 5.0 Δ Δ 0 2-3  9A 7 0.5 0.159 6.0 0 0 0 2-4 10A 80.5 0.159 8.0 0 0 0 2-4 11A 9 0.5 0.159 10.0 0 0 0 2-4 12A 10 0.5 0.15915.0 0 0 0 2-4 13A 11 1.0 0.318 2.5 ⊙ ⊙ ⊙ 2-4 14A 12 2.0 0.637 1.25 0 00 2-3 15A

[0047] In the above examples, the SEM pictures were taken by the productS-4100 made by Hitachi Corp. of Japan. The operation conditions were:23° C., 10-9 torr. The acceleration voltage and the amplification ratesare all labeled on the SEM pictures.

[0048] In electrolysis, the working electrode was anode and made oftitanium. The counter electrode was cathode and made of 316 stainlesssteel. The surface area of the counter electrode is greater than theelectrolysis area of the working electrode.

What is claimed is:
 1. An orthopedic implant having a plurality of poreson at least a portion of its surface, wherein part of the plurality ofpores have a pores-within-a-pore porous structure, saidpores-within-a-pore porous structure comprising a first-stepped pore onsaid surface, and at least one second-stepped pore within thefirst-stepped pore, and optionally a n-stepped pore within a(n−1)-stepped pore, wherein n is third to sixth, wherein saidfirst-stepped pore has an pore opening ranging in size between 10 and800 microns.
 2. The orthopedic implant as defined in claim 1, wherein atleast 30% of said plurality of pores have said pores-within-a-poreporous structure, and said second-stepped pore has an pore openingranging in size between 10 and 500 microns.
 3. The orthopedic implant asdefined in claim 2, wherein at least 50% of said plurality of pores havesaid pores-within-a-pore porous structure, said first-stepped pore hasan pore opening ranging in size between 50 and 600 microns, and saidsecond-stepped pore has a pore opening ranging in size between 50 and400 microns.
 4. The orthopedic implant as defined in claim 2, whereinsaid first-stepped pore having an opening ranging in size between 100and 500 microns, and said second-stepped pore has a pore opening rangingin size between 50 and 300 microns.
 5. A process for making anorthopedic implant, comprising the following steps: forming a pluralityof initial pores on at least one portion of a surface of an orthopedicimplant; and forming a pores-within-a-pore structure within theplurality of initial pores, said pores-within-a-pore porous structurecomprising a first-stepped pore on said surface, and at least onesecond-stepped pore within the first-stepped pore, and optionally an-stepped pore within a (n−1)-stepped pore, wherein n is third to sixth,wherein said first-stepped pore has an pore opening ranging in sizebetween 10 and 800 microns, wherein an intermediate electric currentdensity pore etching method is used to form said pores-within-a-porestructure.
 6. The process as defined in claim 5, wherein said initialpores have a size ranging between 0.2 and 500 microns.
 7. The process asdefined in claim 6, wherein said initial pores have a size rangingbetween 1 and 100 microns.
 8. The process as defined in claim 5, whereinsaid forming said plurality of initial pores on at least one portion ofsaid surface of said orthopedic implant process is carried out by a highelectric current density pore etching method.
 9. The process as definedin claim 8, wherein said high electric current density pore etchingmethod comprises submerging the orthopedic implant in an electrolytesolution to carry out electrolysis by an electric current density higherthan 0.2 A/cm² for a period of 30 seconds to 5 minutes.
 10. The processas defined in claim 9, wherein the electrolyte solution contains 2-5% byweight of NaCl; wherein said current density is at least 0.3 A/cm²; andwherein said electrolysis is carried our for a period of 1 and 3minutes.
 11. The process as defined in any one of claims 5 to 10,wherein said intermediate electric current density pore etching methodcomprises submerging the orthopedic implant in an electrolyte solutionto carry out electrolysis by an electric current density ranging between0.1 and 0.8 A/cm² for a period of 3 to 30 minutes.
 12. The process asdefined claim 11, wherein the electrolyte solution contains 2-5% byweight of NaCl; wherein said current density ranges between 0.2 and 0.7A/cm²; and wherein the electrolysis time ranges between 5 and 20minutes.